The operational (differential) amplifier is today the most widely used analog subassembly, see the article entitled "Operational Amplifiers," Linear Products Data Book, Norwood, Mass., Analog Devices Inc., pp. 2-4 (1988). Electronic differential amplifiers are usually limited to bandwidths less than 200 MHz. Some optoelectronic differential amplifiers have been developed for GHz signal bandwidths, see, for example, the article by Y. A. Choi et al. entitled "Gigahertz-Bandwidth Optoelectronic Differential Amplifier", Microwave and Optical Text Letters, Vol. 1, pp. 49-51 (1988) and the article by B. L. Kasper et al. entitled "Balanced Dual-Detector Receiver for Optical Communication at Gbit/s Rates", Electronics Letters, Vol. 22, pp. 413-415 (1986).
These previous optoelectronic approaches were based on a balanced heterodyne receiver design. The electrical-to-optical and optical-to-electrical conversion was used to electrically isolate the two input signals. The actual signal subtraction was done electronically. This type of circuit has two disadvantages. First, it requires that the constituent devices be carefully selected so they have matching characteristics. Second, the inductors, capacitors and resistors in the balance network can limit the frequency response of the differential amplifier.
Thus, a continuing need exists in the state of the art for an optoelectronic differential amplifier performing signal subtraction while still in the optical domain which provides for several advantages over previous wideband differential amplifiers, namely, eliminating the need for a balancing network, increasing the usable bandwidth, developing a variable gain mechanism and improving linear and spurious free dynamic range.